Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a substantially uniformly charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image to form a toner image. That toner image is then transferred from the photoreceptor onto a copy substrate, such as a sheet of paper. The transferred toner image is then fused to the copy substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process for each color of toner that is used to make the composite color image. For example, in one color marking process a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed to produce a black toner image which is then transferred onto a copy substrate. The photoreceptor is then recharged, re-exposed, and re-developed using a second color of toner, say yellow. That second color image is then transferred onto the copy substrate. The recharge, expose, develop, and transfer process is then repeated for a third color, say magenta, and finally for a fourth color, say cyan. The various color images are placed in superimposed registration such that a composite color image results. That composite color image is then fused onto the copy substrate.
The process of transferring charged toner particles from an image bearing member, such as a photoreceptor, onto a copy substrate involves overcoming the cohesive forces that hold toner particles to the image bearing member. One method of overcoming the cohesive forces involves the application of high intensity electrostatic fields in the transfer region. By controlling the electrostatic fields, the charged toner particles are transferred onto the copy substrate. Complicating the transfer process is the fact that the interface between the image bearing member and the copy substrate is seldom optimal. To improve that interface, mechanical devices that force the copy substrate into intimate and substantially uniform contact with the image bearing surface have been used. The mechanical devices frequently include a blade, referred to herein as a transfer blade, that pushes on the backside of the copy substrate. Typically, the blade force was provided by a spring or some other flexible component.
Unfortunately, prior art mechanical transfer assists have their problems. For example image deletions (caused by insufficient transfer of toner) all too frequently occur. Such image deletions are especially prevalent when duplex printing (marking on both sides of the copy substrate), when marking in color, and/or when the copy substrate changes, such as when papers of different weights are used as copy substrates. While some prior art mechanical transfer assists include electromechanical devices that change the force on the backside of the copy substrate, the force changes are crude and cannot be accurately controlled. Furthermore, prior art mechanical transfer assists are not operator adjustable. Finally, except for difficult to perform mechanical adjustments, prior art mechanical transfer assists do not provide service personnel with the ability to fine tune the operation of the mechanical transfer assist device.
Therefore, a method and apparatus for assisting transfer so as to reduce image deletions would be beneficial. Even more beneficial would be a method and apparatus for assisting transfer that is operator controlled, capable of being tuned by service personnel, and/or that automatically compensates for substrate types and weights.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 5,568,238 to Osbourne et al. discloses an apparatus which transfers a developed image from a photoconductive surface to a copy sheet. The apparatus includes a corona generating device arranged to charge the copy sheet for establishing a transfer field that is effective to attract the developed image from the photoconductive surface to the copy sheet and a blade which is moved from a non-operative position spaced from the copy sheet, to an operative position, in contact with the copy sheet for pressing the copy sheet into contact with at least the developed image on the photoconductive surface to substantially eliminate any spaces between the copy sheet and the developed image during transfer of the developed imaged form the photoconductive surface to the copy sheet. The blade is fabricated to include a conductive material for preventing the generation of electrostatic charge on the blade which may create copy quality defects as a lead edge of the copy sheet passes between the blade and the photoconductive surface. A non-varying pressure is applied by the blade to the copy sheet regardless of the copy sheet stock or whether it is a simplex or duplex copy.
U.S. Pat. No. 5,539,508 to Piotrowski et al. discloses an apparatus for transferring a developed image from a photoconductive surface to a copy sheet. The apparatus includes a continuously variable length contact assembly which is moved from a nonoperative position spaced from the copy sheet, to an operative position, in contact with the copy sheet for pressing the copy sheet into contact with the developed image on the photoconductive surface to substantially eliminate any spaces between the copy sheet and the developed image during transfer of the developed image from the photoconductive surface to the copy sheet. The transfer apparatus is provided with a continuously variable length for selectively corresponding the length of the contact assembly with the process width dimension of the copy sheet.
U.S. Pat. No. 5,300,994 to Gross et al. discloses a transfer system including a contact member for applying pressure against a copy substrate to create uniform contact between the copy substrate and a developed image on an imaging member. The transfer system includes a flexible transfer assist blade and a rotatable cam shaft having a lobe for deflecting the transfer assist blade into contact with the copy substrate. Alternatively, the transfer assist blade may include multiple segments and the rotatable cam shaft may include a plurality of lobes, each having a length wise dimension corresponding to predetermined segments of the blade for providing contact across a dimension corresponding to that of the copy substrate. The system further includes a stepper motor for rotating the cam to predetermined angular positions to create an abutting relationship between the lobe and the transfer assist blade for deflecting selected segments of the blade toward the copy substrate. The transfer assist blade presses the copy sheet into contact with at least the developed image on the photoconductive surface to substantially eliminate any spaces or gaps between the copy sheet and the developed image during transfer of the developed image from the photoconductive surface to the copy sheet.
U.S. Pat. No. 5,300,993 to Vetromile discloses an apparatus which transfers a developed image from a photoconductive surface to a copy sheet. The apparatus includes a corona generating device arranged to charge the copy sheet for establishing a transfer field that is effective to attract the developed image from the photoconductive surface to the copy sheet and a blade which is moved from a non-operative position spaced from the copy sheet, to an operative position, in contact therewith. The blade presses the copy sheet into contact with at least the developed image on the photoconductive surface to substantially eliminate any spaces between the copy sheet and the developed image during transfer of the developed image from the photoconductive surface to the copy sheet.
U.S. Pat. No. 5,678,122 to Gross discloses a transfer system including an adjustable electromechanical operated pretransfer paper guide and a paper basis weight sensor. The pretransfer paper guide moves to a predetermined position to provide a proper bend to a substrate so as to flatten it out as the substrate reaches the transfer station. The position is changed so as to be optimal for both heavy as well as light weight substrates.
U.S. Pat. No. 4,947,214 to Baxendall et al. discloses a transfer assist blade that moves from a nonoperative position spaced from the substrate to an operative position that is in contact with the substrate. The blade presses the substrate into contact with the developed image on a photoreceptor to substantially eliminate any spaces between the substrate and the developed image.